RCU writer side should use rcu_dereference_protected() and not
rcu_dereference(), fix that. This also removes the "suspicious RCU usage"
warning seen when running with CONFIG_PROVE_RCU.

Also, don't use rcu_assign_pointer/rcu_dereference for pointers
which are invisible beyond the udp offload code.

Fixes: b582ef09 ('net: Add GRO support for UDP encapsulating protocols')
Reported-by: NEric Dumazet <edumazet@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: NOr Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: NShlomo Pongratz <shlomop@mellanox.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>